A DRYING TUNNEL FOR FRUIT AND VEGETABLES

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DEM - Demonstration contracts

Obiettivo

To demonstrate the technical and economic feasibilities of using solar energy to dry fruit. Although this demonstration concentrates specifically on drying apples, the drier is a multipurpose unit suitable for any other fruit and possibly even for medicinal or aromatic herbs. It is sited at the "La Vallee du Cady" fruitgrowers' cooperative.The solar installation did work well and the air collectors did not show any major problems. To run collectors in a decompression mode has the advantage that air leaks only lower the temperature but no actual heat losses occur.The main problems for the drying process are the financial and marketing problems. The main costs of the final product are those of the fresh fruit and the labour involved. The marketing of the final product was not solved so that the project, even being totally installed and functioning, was not used by the people of the fruit cooperative: they were not able to sell dried fruit. The installation was never run continuously over several months.Although flow rate was weak, the solar installation successfully maintained the high air temperature required for drying.Total annual incident solar energy on the absorbers was 24,088 Kwh. The installation is capable of supplying 12,000 Kwh/y on continuous working. Of this 12.5% or 1500 Kwh is used for ventilation equally distributed to collectors and the dryer. A ventilator is required since, with natural connection the air passes back and further from one part of the absorber to the other.The use of polished zinasol was problematic. Price, durability and optical characteristics make it an excellent absorber; it is however very sensitive to fluage. As a result, when heated to high temperatures it should be mounted on a rigid structure between supports less than 20 cm apart. Otherwise it will bend as much as 4-5 cm after several weeks of exposure. Energy consumption was high during the first trials as a result of three problems:1. pipes for air descent, the blower and heating battery were not yet insulated;2. no re-cycling was carried out;3. loading timing and duration were not ideal. Consumption did improve during later trials.Drying apricots, after treatment with sulphur to prevent enzymatic browning and preserve colour, was found to be efficient and a sulphuring cupboard was adapted to fit the size of grids in the dryer to facilitate the process.Drying of apples was less successful and percentage losses were high due to poor quality: the apples had been in refrigeration for more than 6 months. In addition, the apples were over-dried to an average moisture content of 8.2%. If inexpensive measuring equipment had been available, this could have been remedied.Overall, the economic advantage of using the solar installation was not demonstrated. Due to the high cost of the solar collectors the installation appears to be no cheaper than electrical energy. In addition, energy costs were found to play a less important role in the economies of fruit drying than the cost of the fruit, wages and packing costs.The hot-air generator consists of 24 m2 rooftop air-type solar collectors, tilted 50 degrees facing south west. The collectors are of the "open loop" type, the absorbers are made of oxidized zinc and the cover plate consists of 4 mm toughened glass. A 12 kw auxiliary electric power supply is connected downstream of the ventilator (flow rate: 1 000 m3 per hour); it is controlled by a solid-state relay system.The air is sucked inside the hall under the roof, achieving a degree of pre-heating. A ventilator sucks the air through the air collectors forcing it through the drying funnel. According to air humidity after the funnel a by-pass recycles part of it.The funnel is 8 m long. The fruit on trays is put on a conveyor belt. Batch drying or continuous drying is possible. The funnel and the tracking mechanics were built by the services of the University of Perpignan. The drying was mainly of apples.